To all the people having problems with i18. The easiest way is probably to set the 'language' property in the yii config (/config/main.php).
For me (language=de) this comes with the correct translation already.

I managed to show seconds... It was a little trouble because even if the locale was set correctly, the widget wasn't displaying them! At first you need the showSeconds: true option, but sadly that's not enough! The full widget code is this:

<?php$yii=dirname(__FILE__).'/../../yii/framework/yii.php';
$config=dirname(__FILE__).'/../protected/config/main.php';
require_once($yii);
// remove the following lines when in production modedefined('YII_DEBUG')ordefine('YII_DEBUG',true);
// specify how many levels of call stack should be shown in each log messagedefined('YII_TRACE_LEVEL')ordefine('YII_TRACE_LEVEL',3);
Yii::setPathOfAlias('estensioni', dirname(__FILE__).'/../protected');
Yii::import('estensioni.extensions.CJuiDateTimePicker.CJuiDateTimePicker');
?>